Objective:
Objective 1: Maximize N use efficiency and animal performance by determining the optimal levels and qualities of dietary protein appropriate for differing base forages in dairy cattle diets, and determining the influence of polyphenol (o-quinones, tannins) or other feed additives on feed N use efficiency.
Sub-objective 1.A. Investigate the factors affecting optimal levels of protein in diets based on different forage types.
Sub-objective 1.B. Determine the influence of dietary polyphenols on efficiency of nitrogen use for milk production.
Objective 2: Determine the relationships between ruminal microbial communities, animal genotype, and/or methane production with feed/nutrient use efficiency and/or lactation performance in response to varying nutritional regimens in beef or dairy cattle.
Objective 3: Determine how the interactions among dietary components influence product formation by ruminal microbes and implications for effects on digesta passage from the rumen, in order to optimize meeting animal nutrient requirements and enhancing animal performance.
Sub-objective 3.A. Determine the quantitative effects of nonfiber carbohydrates and protein on in vitro fermentation kinetics and substrate-to-fermentation end-product conversions.
Sub-objective 3.B. Investigate the effects of interactions among dietary components on lactating dairy cattle.
Sub-objective 3.C. Evaluate the influence of dietary components and characteristics on amounts, proportions, and passage of ruminal liquid digesta.
Sub-objective 3.D. Improve in vitro ruminal fermentation to optimize yield, product concentration and VFA ratios.
Objective 4: Evaluate residual feed intake (RFI), or other measures of nutrient use efficiency, as a measurement and selectable trait for feed efficiency in dairy heifers and lactating dairy cattle and identify and characterize genetic and physiological factors contributing to its variation.
Objective 5: Determine the relationship between measures of nutrient use efficiency in dairy heifers and subsequent nutrient use efficiency as lactating cows; including the evaluation of selection for improved nutrient use efficiency during heifer development on reproduction, lactation performance, stayability, health and milk traits in the lactating cow for potential development of estimated breeding values.

Approach:
Sub-objectives 1.A and 1.B: Latin square feeding studies with lactating dairy cows will be performed to test the effects of: 1) different combinations of dietary forage and supplemental protein sources, and 2) the interaction of dietary tannins and crude protein level of the diet as they influence milk production and efficiency of nitrogen use for milk production. Omasal sampling will be performed using triple markers in order to quantify differences among dietary treatments in flow of amino acids (AA) from the rumen (~metabolizable AA supply to the animal). Effects of tannin and protein levels on nitrogen volatilization will also be evaluated using manure samples from this study. Sub-objective 2.A: Studies will explore the relationship of ruminal microflora profile and milk fat depression (MFD) in lactating dairy cows. 1) Cultures of Megasphaera eldenii strains isolated and enriched from cows displaying or not displaying MFD will be used to determine if they differentially metabolize linoleic acid to produce biohydrogenated fatty acids implicated in MFD. 2) Ruminal inocula collected from lactating dairy cows displaying or not displaying MFD on diets designed to induce MFD. M. elsdenii will be quantified in the archived ruminal samples, and ARISA will be used to identify samples containing unidentified bacterium AL383 which has been shown to be negatively correlated with M. elsdenii. The intent is to identify AL383, isolate it, and assess its interactions with M. elsdenii. Sub-objective 2.B: The impact of interactions of cow genome, lactation performance, and accrual of disease events over multiple lactations will be investigated using records of 4000 genotyped cows. Phenotypic data will be used to establish heritability of phenotypes, and adjust phenotypic data for effects of age on increased risk of decreased performance/increased treated disease events. Phenotypic and genotypic data will then be subject to a genome-wide association analysis. Sub-objective 3.A: In vitro fermentations will be used to investigate relationships among nonfiber carbohydrate sources and level and type of protein supply as they alter the profile, amount, and rate of fermentation product formation by ruminal microbes. Batch culture fermentations of less than 8 hours with multiple destructive sampling times will be used to define patterns of substrate disappearance and microbial product appearance. Sub-objective 3.B: The impact of the protein x carbohydrate interactions described in 3.A combined with influence of changing rates of liquid passage and forage sources will be investigated in studies with lactating cows. Latin square studies with 2 x 2 factorial arrangements of dietary protein solubility and a salt concentration will be performed with repeated ruminal sampling to describe rumen function, omasal sampling to determine AA flow from the rumen, and lactation performance measurements. Sub-objective 3.C: A series of in vivo studies with lactating cows will be conducted to explore the effects of dietary components (salts, soluble protein) on ruminal digesta liquid and dry matter (DM) proportions, total digesta weight, liquid passage rate, and water intake.